Process for making plywood



United States Patent 3,137,607 PROCESS FOR MAKING PLYWOOD Irving S. Goldstein, Pittsburgh, and William J. Oberley, Monroe Heights, Pitcairn, Pa., assignors to Koppers Company, Inc, a corporation of Delaware Filed Aug. 8, 1960, Ser. No. 48,034 3 Claims. (Cl. 156-281) This invention relates generally to the production of plywood.

Plywood, when compared to other wood products, has greater uniformity in strength properties along the direction of its two major axes, has a greatly reduced tendency to twist and warp, is more dimensionally stable with changes in water content, and is more resistant to checking and splitting. Because it has these superior characteristics, several billion square feet of plywood are used each year, and each year finds greater amounts being used than the subsequent year.

Plywood is made up of layers (plies) of thin layers of wood, the plies being joined with an adhesive. To produce plywood, logs are first sawed to the required length and debarked. The logs are then peeled on rotary lathes to produce the layer (veneer of ply) whose thickness usually ranges from to A of an inch. The veneer is then dried, adhesive is applied to the veneer, and the veneer is then laid up in the particular type of construction desired. Generally, the plies are assembled so that the grain of one ply is approximately at right angles to the grain of the next ply; an odd number of plies almost always being used. The assembly is subjected to pressure applied by way of screw, clamp or hydraulic presses for the desired length of time to cure the adhesive.

The use of pressure in the curing assures maximum contact of faces of adjacent layers of veneer and thus decreases the volume of voids without unduly increasing the density of the finished product. Increasing the density of plywood by compression is in general undesirable. So that the density of the panel will be increased less than 10% and preferably 5% or less, the most compressable woods are pressed at lower pressure and the least compressable woods at higher pressure. As would be expected, higher pressures produce the most satisfactory adhesion. The actual amount of compression loss is dependent upon conditions such as moisture content, specific gravity of the wood, and the temperature and pressure applied to the assembly.

The loss of 5-15% of the volume of the wood during the pressing operation presents a problem of great economic importance. Previous attempts to prevent this loss in volume have relied upon mechanical devices such as limit control instrumentation for effecting compression control. A summary of the problems involved, the variables involved, and the mechanical ways of preventing loss in volume has been reported by M. D. McDonald, The Compression of Douglas Fir Veneer During Pressing, J.F.P.R.S. 1, 103-114, 1951.

It has now been found that by impregnating the veneer which is used in plywood with salts before pressing of the plywood, the compression strength of the treated veneer is increased and thereby the amount of compression suffered by the veneer under any given pressing condition is reduced.

3,137,607 Patented June 16, 1964 Wood has been impregnated heretofore, for example, to improve the termite and decay resistance. The effect of preservative salts on the strength properties of wood, particularly on the possible degradation of wood by these materials has been of great concern. The impregnation of wood with chemicals to improve the decay and termite resistance has involved only small amounts of the toxic materials, and these small retentions of salt have not had a marked effect on the compression strength of the wood. The well known text Wood Preservation, by Hunt and Garrett, McGraw-Hill, New York, 1953, observed on page 368, A claim often made in patents for preservatives or in literature promoting them is that the chemicals involved harden the wood. As a general rule, this has no basis in fact and can be ignored unless the process actually provides for the injection of a considerable quantity of the hard material into the wood. Surprisingly, it has now been found that in accordance with this invention, the impregnation of wood veneer with an aqueous solution of a relatively soft material increases the compression resistance of the Veneer.

In accordance with this invention, the veneer may be impregnated with materials such as dicyandiamide and phosphoric acid (as described in US. Patent No. 2,917,- 408), sodium chloride, chromated salts such as chromated copper arsenate, chromated zinc arsenate, and chromated zinc chloride (the chromated salts are physical blends of salts whose formulations are given in the American Wood Preservers Associataion Manual of Recommended Practice Standard P5-56 and which are known commercially as Erdalith, CZC, and Boliden salts), tanalith salts comprised of mixtures of sodium fluoride, sodium chromate, and disodium arsenate such as sold under the trademark Wolman, and sucrose. Other useful materials include copper sulfate, sodium fluoride, ammonium phosphate, ammonium chloride, ammonium sulfate, borax, and boric acid. These materials are advantageously made up as a strong aqueous solution containing from 10 to 50% of the salt, desirably a solution containing about 20% of the material. In accordance with the invention any salt which is soluble in water, which does not destroy the wood, and which can be impregnated readily into the wood to provide for 8 to 40% retention of the material in the wood is satisfactory.

The impregnation of the veneer with the aqueous solution may be carried out by any of the known methods. However, the impregnation may also be carried out by soaking the veneer in the solution, i.e., by the diffusion technique. The veneer could also be treated by dipping the veneer in the solution, and dead-piling the dripping wet sheets of veneer. In the full cell procedure, a high vacuum is applied initially to the pressure vessel containing veneer to remove as much air and free material as possible from the wood. Then, while the vacuum is maintained, the impregnation solution is added to the pressure vessel to surround the veneer. Thereafter, positive pressure is applied to force the impregnant into the veneer. At the end of the impregnation the pressure is released, and the unadsorbed solution removed from the vessel. A vacuum is again applied so as to rapidly remove the solution that would normally run out of the wood and thereby prevent or minimize bleeding.

The veneers so impregnated are then removed from the cylinder and dried. The drying may take place in a conventional manner. For example, the veneer may be placed in multiple tiers of 6 to 8 plies on continuously moving rolls or belt and passed through a tunnel-type drier that operates at a temperature from 303-360 F. Advantageously, the wood is dried until the moisture remaining is within the range of 3 to but it may be as high as 10%.

The dried veneer is ready for assembly or laying up in a conventional manner, the application of adhesive thereto, and the pressing and curing to form the plywood panel. A conventional operation may involve applying to the plies to the extent of about thirty pounds per thousand square feet at each glue line an adhesive which may be a liquid resin having approximately 40% resin solids and about filler, assembling the plies, and then subjecting the assembly to a presure of from 125-15O pounds per square inch if the veneer be from low density wood such as spruce or bass wood, a pressure of 15 0175 pounds per square inch if the veneer be from a. medium density wood such as poplar or gum, and from 200260 pounds per square inch if the veneer be from a high density wood such as birch, maple, or oak. The temperature of the press platen may range from 260300 F. and the time for pressure may range from 1 to 2 minutes.

The invention will be illustrated further by the following examples.

Example I Six samples with dimensions of A X 5 x 5 inches were prepared from a commercial Douglas fir veneer that had the dimensions of A X 48 X 96 inches. Half of the samples were given a full cell treatment in accordance with Example I of Patent No. 2,917,408 by subjecting the samples in a cylinder to a vacuum of -26 inches Hg followed by a two-hour pressure impregnation of the samples at C. and under 160 p.s.i. with an aqueous solution containing 20% by weight of combined dicyandiamide and phosphoric acid in equimolar ratio. These samples, along with the untreated veneers, were placed in a forced convection oven to dry by heating samples at 50 C. for 19 hours, at 70 C. for two hours, and at 90 C. for two hours. The treated samples were then weighed. The dicyandiamide-phosphoric acid retention was 47% on the dry Weight of the wood. Plywoods were prepared from the treated and untreated veneers by bonding three veneer samples together with the grain of each perpendicular to the grain of the adjacent veneer. Casco- Resin glue (WW-17) catalyzed with 0.25% NH Cl, was spread on the contact surface of each outer ply at the rate of 70 pounds glue per thousand square feet of surface. The veneers were pressed at 150 p.s.i., for eight minutes at 110 C. These samples were trimmed to five inches square and the surfaces were sanded to a smooth finish.

It was observed that the surface of the sample from the treated veneers was much smoother than the surface of the sample from the untreated veneers. It is believed this was due to the permanent swelling and salt retention. The application of glue to the treated veneers was much easier and smoother than to the untreated veneers. During the pressing operation, the exudation of glue was noticeably greater from the treated samples, indicating that less glue might be needed in fabricating treated veneers than in fabricating untreated ones.

In the pressing operation, it was observed that pressure loss due to dimension changes in the wood was greater for the untreated samples. Evidence of this property was found when the thickness of the treated and untreated plywood samples were compared. Although the original thickness of the veneers were the same, the samples fabricated from the treated veneers were 20% thicker than those fabricated from the untreated veneers.

Example Il Veneer samples of Douglas fir were dried at C. to dry weight and treated by the full cell procedure as in Example I with 20% solutions of dicyandiamide-phosphoric acid, NaCl, sucrose, CZC, Erdalith, and Wolman salts. However, the dinitrophenol and some of the other salts in the Wolman salt solution were insoluble and the salt concentration achleved was less than 20%. The salt content of these samples 1s shown below.

Thickness Percent Cured Sample No in Inches Treating Salt on Thickness When Dry Solution Dry Wt. (in.)

1 0. 124 45. 3 0.125 2 0. 123 40. 4 0. 125 3 0.124 36. 2 0.127 a 0.123 39. 8 0.123 5 0.122 as. 4 0.123 0.123 41. 5 0.123 0.122 34. 4 0. 125 0.125 35. 9 0. 126 0.123 34. 3 0.125 0.122 49. 4 0.126 0. 123 48. 2 0. 125 0. 122 48. 7 0. 125 0. 124 42. 8 0.124 0. 123 43. 1 0. 123 0.123 43. 3 0. 124 0. 123 as. s 0. 123

man Saits 1 Due to insoluhility ofdinitrophenol and other salts this concentrnatio is less than 20%.

A Carver Laboratory Press containing electrically heated platens and capable of developing a pressure of 6000 pounds with a ram of 1% inch diameter was used to measure the crushing strength of the veneer samples. The platens were heated to a temperature of C. and a test piece three plies thick was placed in the press. The

platens were closed so that the top and bottom veneers were in contact with one of the platens. A dial indicator gage was placed on the movable bottom platen. After a zero point had been set, hydraulic pressure was applied manually by means of a pump so that the bottom platen rose at a constant rate. The pressure was recorded every 0.003 inch that the bottom platen was raised until the crushing strength at proportional limit was exceeded.

FIGURE 1 shows the crushing strength at proportional limit of untreated Douglas fir veneer containing 45% Erdalith salts at 11.7% moisture content. The temperature was maintained at 110 C. for all tests. The figure shows that the crushing strength increases with a decrease in moisture content (curves A and B) and that salts cause an increase in crushing strength at the same temperature and moisture content (curves B and C).

Example III Green Douglas fir veneer having a thickness of A; inch was cut into specimens having areas ranging from 4 to 16 square inches. The samples were dried at 105 C.

Samples were treated by the full cell process in accordance with the procedure of Example I with dicyandiamide-phosphoric acid, chromated zinc chloride (CZC), Wolman salts, sucrose, and sodium chloride. The samples were then dried at 105 C. to about 10% moisture, about 5% moisture, and 0% moisture. The compression loss was measured in accordance with the process of Example II at 25 C. and at 110 C.

Sample No Percent Salt Percent Temp. in Crushing Retention 1 Moisture at Test Strength 1 32 D21 0 110 650 34 Sucrose..-" 0 110 650 49 CZC 4 0 110 550 0 110 750 1 On above dry weight basis in pounds per square inch.

2 Proportional limit.

3 D:Pdicyandiamide:phosphorio acid.

4 GZC-chromated zinc chloride.

5 Wolman-sodium fluoride, arsenate, chromate, and dinitropheuol.

At zero moisture content all of the treated specimens have higher crushing strengths than the untreated samples, although this difference decreases with increased temperature. When the moisture content is increased to about 5% the crushing strength of the salt-treated veneers at both temperatures is greater than the untreated ones, but the difference is less than the difference at zero moisture. When the moisture content is increased to about at a temperature of 25 C., the difference between the treated and untreated specimens decreases; and this difference appears to become even less, when the temperature is increased to 110" C., but the uncertainty in the values is greater than the difference between them; this uncertainty is the result of the variability in the veneer, and the method of measuring. Nevertheless, this chart shows differences and these small differences are large enough to prevent compression deformation in the fabrication of plywood. The loss in moisture of the specimens while being tested at 110 C. was not taken into consid eration and could explain the wide variation in results.

Example IV The procedure of Example III was carried out as before with a dicyandiamide-phosphoric acid solution and with a sodium chloride solution. The samples were dried to a moisture content of 5% and then were subjected to a compression force as in Example II except that the temperature was 25 C. FIGURE 2 shows graphically the results of this example.

Example V The procedure of Example III was carried out with solutions of dicyandiamide-phosphoric acid, sucrose, chromated zinc chloride (CZC), and Wolman salts. The samples were dried until no more free water was present. Thereafter the samples were subjected to compression in the manner described in Example II while a temperature was maintained at 110 C. The results are shown in FIGURE 3.

Example VI Green Douglas fir veneer inch thick was cut into samples 25 square inches in area. The samples were then placed in a 20% solution (as in Example I) of dicyandiamide-phosphoric acid (D:P) maintained at 50 C. and allowed to remain in the solution at atmospheric pressure to allow diffusion of the salt into the veneer to take place. The samples were then removed from the bath and the salt content calculated after oven drying at 105 C. and weighing.

After 45 minutes of submersion the veneers had picked up 9.9% of the weight of the wood in salt by diffusion, and after two hours 16.2%.

Plywood was fabricated from the veneers containing 9.9% salt and untreated veneer by the procedure in EX- ample 1. Although the original thickness of the veneers were the same, the samples fabricated from the treated veneers were 5% thicker than those fabricated from the untreated veneers.

While the foregoing examples have described tests with commercial Douglas fir veneer, it has been found that similar results are obtained with other woods such as spruce, bass wood, poplar, gum, birch, maple, and oak.

We claim:

1. A process for making plywood having a reduced loss in volume during the application of pressure to veneers which comprises subjecting veneers to a vacuum and thereafter, without releasing the vacuum, immersing the veneers in an aqueous solution of salt, applying a positive pressure to the salt solution so as to impregnate the veneers with said solution so that the veneers will retain salt therein to the extent of 10% of its weight, removing the veneers from the solution, applying a vacuum to the veneers so as to remove the material near the surface, drying the veneers, applying adhesive to the veneers and arranging the veneers in an assembly so as to make plywood therefrom, and applying pressure to the assembly until the adhesive sets, whereby the compression resistance of the veneers is increased and the loss in volume of the plywood during the pressing operation is substantially reduced.

2. In a process for making plywood having a reduced loss in volume during the application of pressure to veneers by subjecting veneers to a vacuum, applying adhesive to the veneers at each glue line and arranging the veneers in an assembly so as to make a plywood assembly therefrom and applying pressure to the assembly until the adhesive sets, the improvement which comprises immersing the veneers in an aqueous solution of salt during the application of vacuum to said veneers, applying a positive pressure to the salt soltuion so as to impregnate the veneers with said solution so that the veneers will retain salt therein to the extent of 10 percent of said veneers weight, removing the veneers from said salt solution, applying a vacuum to the veneers so as to remove the material near the surface and drying the veneers prior to the application of adhesive and pressure to the veneers.

3. A process for making plywood having a reduced loss in volume during the application of pressure to veneers which comprises subjecting veneers of A of an inch in thickness to a vacuum of 2526 inches Hg and thereafter, without releasing said vacuum, immersing said Veneers in an aqueous solution containing from 10-50 percent salt selected from the group consisting of sodium chloride, chromated salts, tanalith salts, copper sulfate, sodium fluoride, ammonium phosphate, ammonium chloride, ammonium sulfate, borax, boric acid and dicyandiamide-phosphoric acid, applying a positive pressure of from -275 p.s.i. to the salt solution so as to impregnate the veneers with said salt solution, removing said veneers from the solution, drying said veneers until the moisture remaining within said veneers is within the range of 3-10 percent, arranging said veneers in an assembly, applying adhesive to the veneers to the extent of about 30 pounds per thousand square feet at each glue line, then subjecting said veneers to a pressure of from 125-260 p.s.i. until the adhesive sets, whereby the compression resistance of the veneers is increased thereby substantially reducing the loss in volume of the plywood during the pressing operation.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR MAKING PLYWOOD HAVING A REDUCED LOSS IN VOLUME DURING THE APPLICATION OF PRESSURE TO VENEERS WHICH COMPRISES SUBJECTING VENEERS TO A VACUUM AND THEREAFTER, WITHOUT RELEASING THE VACUUM, IMMERSING THE VENEERS IN AN AQUEOUS SOLUTION OF SALT, APLYING A POSITIVE PRESSURE TO THE SALT SOLUTION SO AS TO IMPREGNATE THE VE NEERS WITH SAID SOLUTION SO THAT THE VENEERS WILL RETAIN SALT THEREIN TO THE EXTENT OF 10% OF ITS WEIGHT, REMOVING THE VENEERS FROM THE SOLUTION, APPLYING A VACUUM TO THE VENEERS SO AS TO REMOVE THE MATERIAL NEAR THE SURFACE, DRYING THE VENEERS, APPLYING ADHESIVE TOTHE VENEERS AND ARRANGING THE VENEERS IN AN ASSEMBLY SO AS TO MAKE PLYWOOD THEREFROM, AND APPLYING PRESSURE TO THE ASSEMBLY UNTIL THE ADHESIVE SETS, WHEREBY THE COMPRESSION RESISTANCE OF THE VENEERS IS INCREASED AND THE LOSS IN VOLUME OF THE PLYWOOD DURING THE PRESSING OPERATING IS SUBSTANTIALLY REDUCED. 